Mark a few fast/canvas web tests as slow.

This cl marks a few fast/convas tests as slow on Mac and/or Debug builds.
Their virtual/gpu equivalents have already been marked as slow.

TBR=fserb@chromium.org
NOTRY=TRUE

Bug: 983642
Change-Id: I2c4ad66a9c073b8f8402710b7d0e87456dfc4e5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760142
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688152}

third_party/blink/web_tests/SlowTests

@@ -685,8 +685,11 @@ crbug.com/980804 virtual/threaded/fast/scrolling/scrollbars/mouse-autoscrolling-
 crbug.com/980804 virtual/main_thread_scrollbar_gestures/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Slow ]
 
 crbug.com/983642 virtual/gpu/fast/canvas/canvas-composite-alpha.html [ Slow ]
+crbug.com/983642 [ Mac Debug ] fast/canvas/canvas-composite-alpha.html [ Slow ]
 crbug.com/983642 virtual/gpu/fast/canvas/canvas-composite-text-alpha.html [ Slow ]
+crbug.com/983642 [ Debug ] fast/canvas/canvas-composite-text-alpha.html [ Slow ]
 crbug.com/983642 virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-e_srgb.html [ Slow ]
+crbug.com/983642 [ Mac ] fast/canvas/color-space/canvas-createImageBitmap-e_srgb.html [ Slow ]
 crbug.com/983642 [ Win ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-p3.html [ Slow ]
 crbug.com/983642 [ Win ] virtual/gpu/fast/canvas/color-space/canvas-createImageBitmap-rec2020.html [ Slow ]